basically just walking around the ship.. like on the first night walk around where the nightclubs are and im sure you'll find people. also in the casino, since you are 18 there is alot of people your age.. also in the disco...
just be friendly!

I was 18 on my last cruise and I kinda just stuck around the teen thing the frisrt night, im sure they wont care or ask your age. But yeah i jsut stuck around and im pretty outgoing so i just introduced myself to a alot of people and made frriends fast...and i never had to go back to the teen meetings that whole week lol.